1. Field of the Invention
The present invention relates to phase shifters for operation at microwave frequencies and more particularly to digital phase bits, which, as elements of a phase shifter, permit the attainment of a series of discrete phase shifts (e.g. 11.25.degree., 22.5.degree. and 45.degree.). The invention further relates to phase bits and phase shifters in which transmission line elements, reactive and resistive devices, and semiconductor switching elements are formed on a common monolithic substrate.
2. Description of the Prior Art
Digital phase bits have been built in solid state form for a number of years using PIN diodes as switching elements. These circuits have been commonly realized in a number of forms such as switched line, switched filter, quadrature hybrid and shunt loaded line designs. More recently SOS (silicon-on-sapphire) and GaAs (gallium arsenide) MESFETS (Metal (gate) Semiconductor Field Effect Transistors) have been used as switching elements in the fabrication of monolithic microwave phase shifter bits. GaAS MESFETs have been used to build switched filter phase shifters while similar devices have been used in switched line configurations. Shunt loaded line designs using MESFETs are also known.
The switched line and switched filter designs mentioned above are inherently lossy structures and the shunt loaded line devices may require good RF grounds which are difficult to achieve in monolithic form to provide proper performance.
A prior phase shifter is disclosed in U.S. application Ser. No. 327,133, filed 12/3/81, entitled "A Continuously Variable Phase Shifter" of R. J. Naster et al and assigned to the Assignee of the present application. In that application, a continuous phase shift is achieved by controlling the conductivity of a field effect transistor shunted with a transmission line.